<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>下拉菜单</title>
    <style>
        *{
            margin: 0;
            padding: 0;
            
        }
        a{
            text-decoration: none;
            
        }
        div {
            width: 1200px;
            height: 30px;
            margin: 0px auto;
            
        }
        li {
            list-style: none;
        }
        div li {
            position: relative;
            float: right;
            text-align: center;
            border: 1px solid black;
            line-height: 30px;
            width: 80px;
            height: 30px;
        }
        div ul {
            position: absolute;
            top:30px;
            display: none;
        }
        div ul li{
            
            float: left;
            width: 120px;
            height: 30px;
            
        }
    </style>
</head>
<body>
    <div>
        <li><a href="#">邮箱</a>
            <ul>
                <li><a href="#">免费邮箱</a></li>
                <li><a href="#">VIP邮箱</a></li>
                <li><a href="#">企业邮箱</a></li>
                <li><a href="#">新浪邮箱客户端</a></li>
            </ul>
        </li>
        <li><a href="#">博客</a>
            <ul>
                <li><a href="#">博客评论</a></li>
                <li><a href="#">来读提醒</a></li>
            </ul>
        </li>
        <li><a href="#">微博</a>
            <ul>
                <li><a href="#">私信</a></li>
                <li><a href="#">评论</a></li>
                <li><a href="#">@我</a></li>
            </ul>
        </li>
        <li ><a href="#">登录</a></li>

    </div>
<script>
    var nav=document.querySelector('div');
    var lis=nav.children;
    for(var i=0;i<lis.length;i++){
        lis[i].onmouseover=function (){
            this.children[1].style.display='block';
        }
        lis[i].onmouseout=function(){
            this.children[1].style.display='none';
        }
    }
</script>
</body>
</html>